L-amino acids such as L-threonine are used in human medicine, in the pharmaceutical industry, in the food industry and, very particularly, in animal nutrition. It is known that L-amino acids can be prepared by the fermentation of strains of Enterobacteriaceae, especially Escherichia coli (E. coli) and Serratia marcescens. As a result of the great importance of these amino acids, efforts are constantly made to improve production methods. Process improvements may relate to fermentation engineering measures, e.g., methods of stirring and supplying oxygen, or to the composition of the nutrient media, e.g., the sugar concentration present during fermentation. Alternatively, improvements may relate to the way in which product is purified, e.g., ion-exchange chromatography, or to the intrinsic performance characteristics of the microorganism itself.
Methods of mutagenesis, selection and mutant choice are often used to improve the performance characteristics of microorganisms. In this way, strains are obtained that are resistant to antimetabolites such as the threonine analog α-amino-β-hydroxyvaleric acid (AHV) or that are auxotrophic for regulatorily important metabolites and which produce L-amino acids such as L-threonine. For some time now, methods of recombinant DNA engineering have also been used for improving L-amino acid-producing strains of the Enterobacteriaceae family. This often involves amplifying individual amino acid biosynthesis genes and testing the effect of this amplification on production. A summary of information relating to the cellular biology and molecular biology of Escherichia coli and Salmonella can be found in Neidhardt (ed.): Escherichia coli and Salmonella, Cellular and Molecular Biology, 2nd edition, ASM Press, Washington, D.C., USA,(1996).